In recent years, air pollution caused by exhaust gas from a diesel engine becomes a social problem. It is the air pollution cause that the amount of nitrogen oxide (NOx) exhausted from the diesel engine per a unit horsepower is larger than that from a gasoline engine, and a large-sized car generally mounts the diesel engine. In the prior diesel engine, power is controlled by only controlling an amount of fuel injection without controlling an amount of oxygen used for fuel burning. In general, the air is introduced in a combustion chamber as large as possible and is caused to react with fuel.
In such a prior art, however, there is a problem which the air excessively introduced in the combustion chamber generates nitrogen oxide when the air is exposed to higher temperature circumstances in an expansion stroke.
Exhaust gas temperature before a muffler, in the usual automobile case, changes widely from 70.degree..about.700.degree. C. or 340.about.970 K. It means that exhaust gas density changes 2.85 times.
For controlling Exhaust Gas Recirculation (EGR) at high temperature precisely, components for EGR control must be protected from higher temperature circumstances. Since exhaust gas temperature changes greatly in a short time, a temperature sensor as an integral component of EGR control is required to response rapidly to wide temperature range. Same problem rises in a method for EGR control.
Exhaust gas density also affects the EGR control. The engine takes in air by the given volume and so the mass recirculation rate for combustion is greatly decreased when recirculation rate is controlled by volume recirculation rate. When recirculation rate is controlled by the mass recirculation rate alternatively, intake of outside air for combustion is greatly decreased. Therefore both are not practical. Said influence can be avoided by controlling exhaust gas temperature to be below 120.degree. C. (390 K) approximately at the junction of recirculating gas flow and incorporated outside air flow.
The throttle valve accuracy degrades when particles in the recirculating gas adhere to the valve, and the accuracy of a pressure sensor of a flowmeter degrades when particles jam the port of the pressure sensor.
It is an object of the invention to provide the method and apparatus for controlling a diesel engine that supplies appropriate amount of oxygen for fuel burning so as to inhibit generation of nitrogen oxide.